Projects at the Radio Communications Research Unit (RCRU)

RUTHERFORD APPLETON LABORATORY

The VLF to HF Bands

Results from the Radio Amateurs
Propagation at 1440 kHz

Radio amateur experiments

The radio amateurs with their more sophisticated radio equipment were in a position to make more systematic observations and cover a much wider band of frequencies. This event was also an opportunity to involve radio amateurs directly with RA and radio researchers at RAL. Scientifically it provided an opportunity to compare the observations during an eclipse with those published by the International Telecommunications Union (ITU) for day-time/night-time differences and to model the consequences of the effects on the ionosphere.

The experiment for the radio amateurs and RA regional offices was essentially a classical A3 ionospheric absorption measurement [Davis 1990, Rawlings 1976]. This is where the strength of a continuous wave (CW) signal from a remote transmitting station is monitored in time using a calibrated signal strength meter. The strength of the signal was expected to increase for most frequencies and propagation paths as the absorbing D layer of the ionosphere disappeared with the loss of sunlight (as shown in Table 1).

Table 1. Examples of some typical changes in signal strength between normal daytime and night-time

A description of the radio amateur and RA experiment

In detail the experiment required noting down the strength of radio signals to and from the Continent, before, during and after the eclipse. That meant between about 8:30am and 13:00pm (British Summer Time) on the 11th August 1999 and similarly on a control day either the day before or after. Some used computers to record the signal levels but many just used pen and paper. One problem, that was identified early on, was that most signal strength meters or dials on radio amateur equipment are likely to be too unreliable to be used directly for measuring received signal strength. However advice was provided in detail as to how to overcome this by supplying a procedure as to how to calibrate these meters into dB. This many, but not all, took the time to do. However even uncalibrated measurements had value. The importance to take great care about the exact timing of the observations was constantly emphasised.

Publicising the experiment to radio amateurs was done at Conferences, through the HF Technical Working Party, speaking at local clubs and publishing articles in radio amateur journals such as RadCom and Radio Today. Laminated certificates were sent to the radio amateurs who sent their results to Rutherford Appleton Laboratory as thanks.

Results from the Radio Amateurs [TOP]

Figure 13 shows just a selection of the responses sent in by the radio amateurs. The frequencies chosen here range from 864kHz to 7MHz. The change in signal strength for these frequencies ranges from 10 to 40 dB. Similar to the ITU day-time/night-time values shown in Table 1 above. In Figure 14, Figure 15 and Figure 16 just three examples of the variations of signal strength in time are shown along side a map of Europe with the locations of the transmitters and receivers in each case. The path the eclipse totality passed is indicated on the map by the thick black line. In the first example (Figure 14) the signal strength variation is for a propagation path which must traverse the path of totality. The propagation paths are shown as simple straight-line projections onto the ground on the map, though the radio signals must have reflected from the ionosphere and in reality follow a 3D path that would require a ray trace to model accurately. The frequency was 6.065 MHz and was recorded in Spain from a transmitter in Sweden. The 100% eclipse shadow passed the mid-point of the propagation path at approximately the same time as the signal strength reached its maximum. A red star indicates this point on both the map and time plot. The slight difference in time is probably due to a combination of the sampling interval and the fact that the eclipse path on the map is shown for the eclipse on the ground. Up at ionospheric altitudes the eclipse occurred a few minutes earlier.

The maximum signal when the eclipse is at the mid-point is the behaviour one would expect. That is although the point of reflection of the radio waves is going to be the E layer above the D layer, the maximum increase in signal strength of a received signal would occur when the average absorption is a minimum on both upward and downward legs of the journey. This is illustrated in Figure 12 below.

Figure 12. A sketch showing a simple view of radio waves reflection off the E layer of the ionosphere. The eclipse 100% shadow is shown as the dark oval in the centre as if it were affecting the D layer. The shaded oval represents the partial eclipse shadow that is affecting the D layer absorption on the upward and downward propagation.

A similar observation was found for most frequencies and propagation paths across and along the path of totality. However there were exceptions which are discussed in a later section . Two examples for the variation in signal strength at 3.522MHz and 864 kHz from a receive site directly under the path of totality in the UK from transmitter just north of the totality are shown in Figure 15 and Figure 16. Here too the maximum signal occurs approximately when the eclipse totality was at the mid-point of the projected straight-line propagation path.

	Figure 14. The variation in signal strength for propagation across the path of totality at 6.065 MHz (Click to enlarge)
	Figure 15. The variation in signal strength for propagation across the path of totality at 3.522 MHz (Click to enlarge)
	Figure 16. The variation in signal strength for propagation across the path of totality at 864 kHz (Click to enlarge)

LF and VLF observations during the eclipse

The LF and VLF band (3kHz -300kHz) data from radio amateurs showed some very interesting effects of the eclipse. The plots in Figure 17, which show the temporal variation in the signal strength at 75kHz from the Swiss time clock transmissions (HBG), illustrate this. These are very different from the responses at the MF and HF (300kHz – 30MHz). The map in Figure 18 shows the different direct line propagation paths between the transmitter and the receiving locations for the signals shown in Figure 17. There is more than one explanation for the oscillations clearly seen at this frequency. Firstly they could be the result of phase changes incurred by the radio signals due to the effects of the eclipse shadow on the ionosphere where the signals are being reflected (this has been seen before). Alternatively changes in the heights of the layers and multi-path interference are two other explanations or a combination of all of these effects. At these frequencies the radio waves are predominately being reflected by the D layer of the ionosphere which is undergoing a lot of changes as a consequence of the eclipse.

Contributions from the RA regional Offices

Staff at the RA Regional Offices and at the Baldock monitoring station also contributed to the radio propagation observations of the total solar eclipse in 1999. A total of 16 computer-controlled radio "scanners" were purchased and distributed to the RA regional offices to make accurately timed observations using computers. The receivers used were ICOM/IC-PCR1000 programmable LF/MF/HF radio receivers controlled by PC with a 10-second sampling interval. Each receiver was calibrated into dBm before deployment at a range of spot frequencies from 250kHz to 15MHz. The antennas used were mainly low noise, broadband active whip antennas (SONY AN-1). These provided a more consistent data set of observations.

Propagation at 1440kHz [TOP]

One of the best transmitter stations to observe turned out to be the 1200 kW transmitter of Radio Luxembourg broadcasting at 1440kHz from Marnach. An example of the signal strength variation on the eclipse day and a control day is shown in Figure 19. The decrease in signal strength due to the increasing levels of absorption in the D layer as the sun rises at dawn is well illustrated on both days between 4am and 6:30am. During the 1.5 hours of the eclipse the signal level can clearly seen to return to 60% of the night-time (top panel). The straight-line projection onto the ground of the propagation path from Marnach where Radio Luxembourg to Birmingham is show as the dashed line in the map in Figure 20 along with paths to 5 other RA regional offices (solid lines) and Chilton, Rutherford Appleton Laboratory (dotted line).

Figure 21, shows the temporal variation the signal strength, similar to the plot shown in Figure 19, but with the signal level for the normal day subtracted to highlight the difference made by the eclipse. This type of plot allows a closer examination of the timing of the effect on the radio reception and the passage of the lunar shadow to be compared for all the receiving stations. The times and the peak values for the all the receiving stations are listed in Table 2. The latitude and longitude of the receiving stations and the great circle ground range between the transmitter and each receiver is also included in the table. The precision of the receiving station clocks was checked manually before and after the time of the eclipse and corrected for.

Figure 21. Variation in received signal at Chilton in the UK of the 1440kHz CW Radio Luxembourg carrier broadcast from Marnach (Radio Luxembourg). Here a 5 minute smoothing has been applied. (Click to enlarge)

Table 2. The results from the UK receivers monitoring the carrier frequency from the Marnach transmitter. The 100 % passed over the Marnach transmitter at 10:28:58 UT [Bell 1990]. * Uncalibrated receiver. Marnach Tx is 49.62N, 6.0 E.

The eclipse totality was directly over Luxembourg at 10:28:58 UT. This is very similar to the time of maximum signal (column 7 in Table 2) rather than the time of local eclipse maximum over the receiver stations (column 5 in Table 2) for all but one case. What this table reveals is the general tendency for the peak signal strength at 1440kHz to occur when the eclipse shadow was much closer to the transmitter than either the path mid-point. This was not the case for HF frequencies. The exception to this is the case for 1440 kHz being received at Helston where the receiver was also directly under the path of totality and the propagation from the transmitter to the receiver would have experienced the maximum effect of the eclipse.

What was most unexpected was that for the signal strength responses are centred on the time the eclipse passed over the transmitter, the enhancement of > 10dBm to the reception in the UK continues when the lunar shadow had progressed well into southern Germany, more than 10 minutes after the maximum.

This suggests that the loss of absorption directly over the transmitter had a more significant effect than the loss of absorption at any other region of the path for this transmitter for these cases at 1440kHz.

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